Method and apparatus for reducing audio noise in a switching regulator

ABSTRACT

A switching regulator utilizing on/off control that reduces audio noise at light loads by adjusting the current limit of the switching regulator. In one embodiment, a switching regulator includes a state machine that adjusts the current limit of the switching regulator based on a pattern of feedback signal values from the output of the power supply for a preceding N cycles of the drive signal. The state machine adjusts the current limit lower at light loads such that cycles are not skipped to reduce the operating frequency of the switching regulator into the audio frequency range until the flux density through the transformer is sufficiently low to reduce the generation of audio noise.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to power supplies and,more specifically, the present invention relates to a switchingregulator.

[0003] 2. Background Information

[0004] Electronic devices use power to operate. Switched mode powersupplies are commonly used due to their high efficiency and good outputregulation to power many of today's electronic devices. In a knownswitched mode power supply, a low frequency (e.g. 50 or 60 Hz mainsfrequency), high voltage alternating current (AC) is converted to highfrequency (e.g. 30 to 300 kHz) AC, using a switched mode power supplycontrol circuit. This high frequency, high voltage AC is applied to atransformer to transform the voltage, usually to a lower voltage, and toprovide safety isolation. The output of the transformer is rectified toprovide a regulated DC output, which may be used to power an electronicdevice. The switched mode power supply control circuit usually providesoutput regulation by sensing the output and controlling it in a closedloop.

[0005] A switched mode power supply may include an integrated circuitswitching regulator, which may include a power switch or transistorcoupled in series with a primary winding of the transformer. Energy istransferred to a secondary winding of the transformer by turning on andoff of the power transistor in a manner controlled by the switchingregulator to provide a clean and steady source of power at the DCoutput. In a known switching regulator, a feedback current is sampledfrom the output of the DC output of the power supply. When the feedbackcurrent is below a regulation threshold, the power switch is switched ata constant frequency. However, when the feedback current is above aregulation threshold, the switching regulator is disabled, resulting ina skipped cycle of the power switch.

[0006] When cycles are skipped by a switching regulator as describedabove, the resulting frequency of operation of the switching regulatoris reduced. Thus, the frequency of operation of the switching regulatoris varied as cycles are skipped to regulate the DC output of the powersupply, with the frequency decreasing as the load coupled to the DCoutput decreases. Generally, when the frequency of operation of knownpower supplies of this type drop to frequencies within the audiofrequency range, such as within 20 Hz to 20 kHz, undesirable audio noiseis generated by the transformers of the power supplies.

SUMMARY OF THE INVENTION

[0007] Switching regulator methods and apparatuses are disclosed. In oneembodiment, a switching regulator includes a power switch coupledbetween first and second terminals. The first terminal is coupled to anenergy transfer element of a power supply and the second terminal to becoupled to a supply rail of the power supply. A drive signal generatorcircuit is coupled to a third terminal to receive a feedback signalrepresentative of an output of the power supply. The drive signalgenerator generates a drive signal coupled to control switching of thepower switch in response to the feedback signal. The drive signalgenerator circuit selectively disables each on period of the drivesignal in response to the feedback signal to regulate the output of thepower supply. A current limit circuit is coupled to the power switch andthe drive signal generator circuit to control the drive signal to limita current flow through the power switch. The current limit circuitincludes a plurality of current limit settings for the power switch thatare selected in response to the feedback signal. Additional features andbenefits of the present invention will become apparent from the detaileddescription, figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention detailed illustrated by way of example andnot limitation in the accompanying figures.

[0009]FIG. 1 is a schematic illustrating one embodiment of a powersupply including a switching regulator in accordance with the teachingsof the present invention.

[0010]FIG. 2 is a schematic illustrating one embodiment of a switchingregulator in accordance with the teachings of the present invention.

[0011]FIG. 3 is a state machine diagram illustrating one embodiment ofthe processing flow between states of a state machine in accordance withthe teachings of the present invention.

[0012]FIG. 4 is a schematic illustrating one embodiment of state machinecircuitry in accordance with the teachings of the present invention.

[0013]FIG. 5 is a schematic illustrating one embodiment of current limitadjust circuitry in accordance with the teachings of the presentinvention.

[0014]FIG. 6 is a timing diagram illustrating waveforms of oneembodiment of switching regulator operating in various states of a statemachine with varying current limit levels in accordance with theteachings of the present invention.

[0015]FIG. 7 is a timing diagram illustrating waveforms of anotherembodiment of switching regulator operating in various states of a statemachine with varying current limit levels in accordance with theteachings of the present invention.

[0016]FIG. 8 is a timing diagram illustrating waveforms of yet anotherembodiment of switching regulator operating in various states of a statemachine with varying current limit levels in accordance with theteachings of the present invention.

[0017]FIG. 9 is a timing diagram illustrating waveforms of still anotherembodiment of switching regulator operating in various states of a statemachine with varying current limit levels in accordance with theteachings of the present invention.

DETAILED DESCRIPTION

[0018] Method and an apparatus for regulating a power supply aredisclosed. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one having ordinary skill inthe art that the specific detail need not be employed to practice thepresent invention. In other instances, well-known materials or methodshave not been described in detail in order to avoid obscuring thepresent invention.

[0019] In one embodiment, a switching regulator in accordance with theteachings of the present invention operates in a manner such that modesof operation in the audible frequency range are avoided. One embodimentof the switching regulator includes a state machine, with each staterepresenting a current limit level. At full load, the current limit isat the full level. As the load decreases, the frequency decreases untilit is approximately 20 kHz, the level at the upper end of the audiblefrequency range. At this point, a state transition to one with a lowercurrent limit is executed. In order to provide the same power to theoutput, the feedback loop will request more switching cycles, thusincreasing the frequency of operation. Therefore, the frequency ismaintained above the audio frequency range at this point. In oneembodiment, this process is repeated as the load is reduced until thestate with the lowest current limit has been reached. This state has acurrent limit level that is low enough such that the flux densitythrough the power supply transformer does not cause the transformer toproduce unacceptable levels of audio noise. Therefore, the flux densitythrough the transformer is limited to low values with the selected lowcurrent limit levels when the switching regulator operates within theaudible frequency range due to light loads.

[0020] In one embodiment, the switching regulator senses a feedbackcurrent that is determined by the status of the regulation of the outputof the power supply. The more the output is below its desired level, thelower the magnitude of this current becomes. As will be shown, if themagnitude of this current is below a set threshold, a digital signalinside the regulator circuit, referred to herein as an Enable signal,will become logic level one and the switching regulator circuit willswitch. If the magnitude of this current is above a set threshold, theEnable signal will become logic level zero and the switching regulatorcircuit will skip a cycle.

[0021] As will be discussed, a state machine and a plurality of currentlimit settings are utilized in accordance with the teachings of thepresent invention. In one embodiment, the state is in a low state atstart-up of the switching regulator. The low state selects a lowestcurrent limit setting of one embodiment of the switching regulator. Ifthis current limit in the low state is insufficient to regulate theoutput, which can happen at start-up or if the load is increased, theregulator circuit will not skip any cycles.

[0022] In one embodiment, after a pattern of N equals six consecutiveEnable digital ones for a preceding N equals six consecutive switchingcycles at the lowest state, the state machine transitions to the amedium state, which corresponds to a medium current limit level. If forsome reason the load is reduced and the regulator circuit encounters Nequals six consecutive Enable digital zeroes, which results in N equalssix consecutive skipped cycles, the state machine makes a transitionback to the low state. This prevents the regulator circuit fromoperating in the audible range in the medium state. In one embodiment,if the current limit in the medium state is insufficient to regulate theoutput, which can happen at start-up or if the load is increased, theregulator circuit will not skip any cycles.

[0023] In one embodiment, after a pattern of N equals six consecutiveEnable digital ones for a preceding N equals six consecutive switchingcycles at the medium state, the state machine transitions to the highstate, which corresponds to a high current limit level. If for somereason the load is reduced and the regulator circuit encounters N equalssix consecutive Enable digital zeroes, which results in N equals sixconsecutive skipped cycles, the state machine makes a transition back tothe medium state. This prevents the regulator circuit from operating inthe audible range in the high state. If the current limit in the highstate is insufficient to regulate the output, which can happen atstart-up or if the load is increased, the regulator circuit will notskip any cycles.

[0024] In one embodiment, after a pattern of N equals six consecutiveEnable digital ones for a preceding N equals six consecutive switchingcycles at the high state, the state machine transitions to a state witha high current limit level, referred to herein as a super high state,but one without any skipping of cycles. In this super-high state, anEnable digital one results in a switching cycle at the high currentlimit level, while an Enable digital zero results in a switching cycleat the medium current limit level. This prevents the skipping of cyclesat a frequency in the audio frequency range. If for some reason the loadis reduced and the regulator circuit encounters N equals six consecutiveEnable digital zeroes, the state machine makes a transition back to thehigh state with the skipping of cycles.

[0025] In one embodiment, the various plurality of current limit levelsand the point of transition from one level to the next are carefullyoptimized. In one embodiment, the state machine in accordance with theteachings of the present invention is designed such that oscillationsbetween states do not occur. If these oscillations occur at asufficiently high frequency, the audio noise problem can reappear. Theseproblems can occur if a load exists such that it cannot be handled byany combination of switching and skipping in any state. For example, theenergy generated by one switching cycle followed by 5 skipped cycles atthe high current limit level can be too much for a certain load. If, atthe same time, the energy generated by one skipped cycle followed by 5switching cycles at the medium current limit level is too little toregulate this same load, then the state machine will oscillate betweenthe two states, possibly causing audio noise. Thus the energy levels ofthe different states overlap in one embodiment. The current limit levelsof the various states are not separated by a substantially large degree.In addition, the number of N cycles of delay required for changingstates is not too small. For instance, N is equal to 6 in oneembodiment. It is appreciated however that in other embodiments, N maybe greater than or less than 6.

[0026] In one embodiment, stability of the state machine is improved toa greater degree while at the same time maintaining the transientresponse at start-up to a heavy load. Increased stability is realized inthis embodiment with the inclusion of yet another current limit levelstate by incorporating hysteretic behavior in the medium state. Themedium current limit level is split into two distinct levels, a lowermedium level and an upper medium level.

[0027] In one embodiment, after power-up when the regulator circuitfirst enters the medium state, the current limit will be set to thelower medium level. If the regulator circuit transitions to the highstate and then back to the medium state, the current limit will be setto the upper medium level. If the state machine receives the pattern ofN consecutive Enable digital zeroes that cause the transition from theupper medium state to the low state and if it then receives theconsecutive Enable digital ones to transition back to the medium state,then the current limit will be set to the lower medium level. The uppermedium and lower medium states in accordance with the teachings of thepresent invention are two different states with different current limitlevels. The benefit of this embodiment is mainly in transient responseas it would take less cycles to move from one end of the states to theother end.

[0028] To illustrate, FIG. 1 is a schematic illustrating one embodimentof a power supply 100 including a switching regulator 139 in accordancewith the teachings of the present invention. As shown, an alternatingcurrent (AC) mains voltage is input through resistor 101 into bridgerectifier 147, including diodes 103, 105, 107 and 109, which provides arectified signal to power supply capacitors 113 that provide input DCvoltage to primary winding 149 of energy transfer element or transformer125. It is appreciated that supply rails are provided at the ends ofbridge rectifier 147. Switching regulator circuit 139 allows current toflow through primary winding 149 during its on state of each switchingcycle and acts as open circuit when in its off state. When current flowsthrough primary winding 149, transformer 125 is storing energy. When nocurrent is flowing through primary winding 149, any energy stored intransformer 125 is delivered from secondary winding 141 to capacitor131. Capacitor 131 delivers power to the load 143. The voltage acrossthe load 143 will vary depending on the amount of energy stored in thetransformer 125 in each switching cycle which is in turn dependent onthe length of time current is flowing through primary winding 149 ineach switching cycle.

[0029] In one embodiment, the sum of the voltage drop across optocoupler127 and the reverse break down voltage of zener diode 133 isapproximately equal to the desired output threshold level across load143. When the voltage across the load 143 reaches the threshold level,current begins to flow through optocoupler 127 and zener diode 133 thatin turn is used to disable the switching regulator circuit 139. In oneembodiment, whenever switching regulator circuit 139 is in the off-statethe regulator circuit power supply bypass capacitor 123 is charged tothe operating supply voltage, which in one embodiment is typically 5.7volts by allowing a small current to flow from bypass terminal 145 tothe switching regulator circuit power supply bypass capacitor 123.Regulator circuit power supply bypass capacitor 123 is used to supplypower to operate switching regulator circuit 139 when it is in theon-state.

[0030] In one embodiment, switching regulator circuit 139 operates inthe following fashion under most loads except with very heavy loadswhich is described later. When the switching regulator circuit 139 isdisabled, an open circuit condition is created in primary winding 149and transformer 125 does not store energy. The energy stored in thetransformer 125 from the last cycle of switching regulator circuit 139is then delivered to secondary winding 141, which in turn supplies powerto load 143 at the output of the power supply 100. Once the remainingenergy in transformer 125 is delivered to the load 143 the voltage ofthe load 143 will decrease.

[0031] When the voltage at the load 143 decreases below the thresholdlevel, current ceases to flow through optocoupler 127 and switchingregulator circuit 139 resumes operation either instantaneously or nearlyinstantaneously. Under very heavy loads, the switching regulator circuit139 in one embodiment operates in a slightly altered fashion. Thecurrent limit level chosen by a state machine included in one embodimentof switching regulator circuit 139 is the highest level under very heavyload. However, the switching regulator circuit 139 will not entirelycease to operate when the voltage at the load is above the thresholdlevel. Instead it will operate at a lower current limit level.

[0032] As mentioned, one embodiment of switching regulator circuit 139includes a state machine that, depending on the load 143, chooses theappropriate current limit level among a discrete and finite number of aplurality of current limit levels. The selected current limit levelturns off the switching regulator circuit 139 when the current flowingthrough the primary winding 149 or switching regulator circuit 139 risesabove the selected current threshold level.

[0033]FIG. 2 is a schematic illustrating one embodiment of a switchingregulator 139 in accordance with the teachings of the present invention.As shown, switching regulator circuit 139 includes a power switch ormetal oxide semiconductor field effect transistor (MOSFET) 229 that iscoupled between a drain terminal 231 and a source terminal 233. MOSFET229 is switched on and off according to a drive signal 249 generated bya drive signal generator. In one embodiment, drive signal 249 is inputinto the gate of MOSFET 229 by AND gate 225. In one embodiment, drivesignal generator includes AND gates 215 and 225, OR gate 217, latch 219,oscillator 207, state machine circuitry 301, current limit adjustcircuitry 305 and their associated elements. The input of AND gate 225includes an output of a latch 219, a bypass terminal voltage indicator257 provided by undervoltage comparator 213, and a thermal status signal241 from thermal shut-down circuit 209. In one embodiment, Maximum dutycycle signal 237 generated by oscillator 207 determines the maximum timethat MOSFET 229 can conduct in each cycle of operation.

[0034] When the phototransistor 127 current being pulled out of thefeedback input 203 is greater than the current source 205, Enable signal235 will be pulled to a low state. When the phototransistor 127 currentbeing pulled out of the feedback input 203 is less than the currentsource 205, Enable signal 235 will be pulled to a high state. As shown,Enable signal 235 is also coupled to be received by the state machinecircuitry 301. State machine circuitry 301 will send signals 303 to thecurrent limit (πim) Adjust circuitry 305, setting the current limit ofI_(drain) 255 through MOSFET 229 or primary winding 149 to be lower inlight load or higher in high load. In one embodiment, there are threesignals 303 a, 303 b and 303 c included in signals 303.

[0035] In one embodiment, current limit adjust circuitry 305 adjusts thecurrent limit in digital steps. Transitions to a higher current limitstate occur after a pattern of N consecutive Enable signal 235 logichighs. Transitions to a lower current limit state occur after a patternof N consecutive Enable signal 235 logic lows. In one embodiment, Nequals 6. At a sufficiently high current limit state, super high signal309 output of state machine circuitry 301 will be set to be logic highstate. As a result, OR gate 313 will set signal 315 to be high wheneither the state machine circuitry 301 is in the super high state orwhen Enable signal 235 is high. Signal 315 ultimately determines whethera switching cycle will occur. Thus, when the state machine circuitry 301is not in the super-high state, the Enable signal 235 determines whetheror not a switching cycle will occur. However, when the state machinecircuitry 301 is in the super-high state and super high signal 309 is ina logic high state, all switching cycles will occur at either one of twodesignated current limit levels.

[0036] In one embodiment, the inputs to latch 219 include an OR gateoutput signal 245 and an AND gate output signal 243. The AND gate outputsignal 243 is high only when signal 315 and clock signal 239 generatedby oscillator 207 are both high. Thus, AND gate 215 provides output whenlogical high signal 315 is received and clock signal 239 is provided byoscillator 207. In operation, when signal 315 is high, the clock signal239 is transferred to latch 219 by the AND gate 215, thereby setting thelatch 219 and enabling that cycle to go through and turn on the MOSFET229. Conversely, when the signal 315 is low, it blocks the clock signalfrom setting the latch 219, and keeps the MOSFET 229 off during thatcycle.

[0037] In one embodiment, OR gate output signal 245 is provided by ORgate 217 when the current threshold limit is reached or during the timewhen maximum duty cycle signal 237 is in an off state. In operation, ORgate output signal 245 is high when either the maximum duty cycle signal237 is low or when the current limit is reached after the leading edgeblanking delay, which is determined by leading edge blanking circuit223, in order to turn off the MOSFET 229.

[0038] In one embodiment, signal 317 generated by current limit adjustcircuitry 305 is a voltage level proportional to the voltage across theMOSFET 229 on-resistance. Current limit states are determined by signals303 a, 303 b and 303 c, which are generated by state machine circuitry301. At higher current limit states, current limit adjust circuitry 305changes signal 317 to become a lower proportion of the MOSFET 229on-resistance voltage. At lower current limit states, block 305 causessignal 317 to become a higher proportion of the MOSFET 229 on-resistancevoltage. Current threshold comparator 227 then compares signal 317 to aset voltage, current threshold limit voltage V_(ILIMIT) 251. If signal317 is above the current threshold limit voltage V_(ILIMIT) 251 thecurrent limit signal is triggered and the MOSFET 229 is turned off andthen will not begin conducting until the beginning of the next on-time.

[0039] In one embodiment, the switching regulator circuit 139 turns offthe MOSFET 229 after the current on cycle when the signal 315 is pulledlow and creates a condition where there will be no additional powersupplied to the load. Accordingly, signal 315 in response to the outputof power supply 100 selectively allows the on time of a current cycle ofdrive signal 249 to be maintained and not allow or disable an on time ofa next cycle of drive signal 249. When signal 315 is pulled high, theMOSFET 229 will resume operation upon the beginning of the nexton-period of the maximum duty cycle signal 237.

[0040] In one embodiment, a bypass circuit or 5.7V regulator 211, whichincludes the current source from the drain terminal 231 to the bypassterminal 145, regulates the power level of regulator circuit powersupply bypass capacitor 123 at a voltage level, which in one embodimentis 5.7 volts. This is done by charging the switching regulator circuit139 power supply bypass capacitor 123 when the MOSFET 229 is notconducting. In one embodiment, undervoltage comparator 213 prevents theMOSFET 229 from conducting again until the voltage at bypass terminal145 reaches the desired voltage level. Inverter 307 is used to invertthe output of an undervoltage comparator 213.

[0041]FIG. 3 is a state machine diagram 351 illustrating one embodimentof the processing flow between states of state machine circuitry 301 inaccordance with the teachings of the present invention. As shown, oneembodiment of state machine diagram includes five states: low state 353,lower medium state 355, upper medium state 357, high state 359 and superhigh state 361. In one embodiment, each state selects from a pluralityof current limit settings for current limit adjust circuitry 305. Table1 below summarizes the current limit settings or cycle skipping settingsselected by the states according to one embodiment of the presentinvention. TABLE 1 State Machine Current Limit Settings STATE ENABLE = 0ENABLE = 1 low skip 0.4 Ilim-max lower medium skip 0.5 Ilim-max uppermedium skip 0.7 Ilim-max high skip Ilim-max super high 0.5 Ilim-maxIlim-max

[0042] As shown in the embodiment summarized in Table 1, when in lowstate 353, a cycle of drive signal 249 is skipped when Enable signal 235is low and the current limit setting is 0.4 πim-max when Enable signal235 is high. When in lower medium state 355, a cycle of drive signal 249is skipped when Enable signal 235 is low and the current limit settingis 0.5 πim-max when Enable signal 235 is high. When in upper mediumstate 357, a cycle of drive signal 249 is skipped when Enable signal 235is low and the current limit setting is 0.7 πim-max when Enable signal235 is high. When in high state 359, a cycle of drive signal 249 isskipped when Enable signal 235 is low and the current limit setting isπim-max when Enable signal 235 is high. When in super high state 361,the current limit setting is 0.5 πim-max when Enable signal 235 is lowand the current limit setting is πim-max when Enable signal 235 is high.Note that in one embodiment, no cycles are skipped in drive signal 249when in super high state 361. It is also noted that in one embodiment,the lower current limit settings, e.g. 0.4 πim-max, result in low fluxdensity through the transformer 125 when switching regulator circuit 139operates at lower frequencies within the audible frequency range (e.g.20 Hz to 20 kHz). As a result, unacceptable audio noise is not generatedby power supply 100 in accordance with the teachings of the presentinvention. Stated differently, a switching regulator circuit 139 inaccordance with the teachings of the present invention will not operatewithin the audible frequency range unless the flux density is limited tobe below a sufficiently low threshold value to reduce the generation ofundesired audible noise.

[0043] As shown in FIG. 3, at power-up the state machine circuitry 301starts at low state 353. The state machine circuitry 301 will stay inthe low state 353 until a pattern of N equals 6 consecutive high Enablesignals 235 occur. In one embodiment, this will be the case when theoutput load 143 is light. The state machine circuitry 301 will move upto lower medium state 355 upon the occurrence of a pattern of N equals 6consecutive high Enable signals 235. This is illustrated in FIG. 3 withtransition 363. The state machine circuitry 301 will stay in this stateunder a medium load 143. If the load 143 is further increased, a patternof N equals 6 consecutive high Enable signals 235 will occur again andthe state machine circuitry 301 will move up to high state 359, andsimilarly to super high state 361. This is illustrated in FIG. 3 withtransitions 367 and 375, respectively. If the load 143 is decreased, thestate machine circuitry 301 will move down upon the occurrence of apattern of 6 consecutive low Enable signals 235 until the appropriatestate is established. For instance, transition 377 illustrates statemachine circuitry 301 changing from super high state 361 to high state359, transition 369 illustrates state machine circuitry 301 changingfrom high state 359 to upper medium state 357 and transition 373illustrates state machine circuitry 301 changing from upper medium state357 to low state 353.

[0044] As mentioned earlier, improved transient response is provided forstate machine circuitry 301 by incorporating hysteretic behavior in themedium state. Indeed, the medium state is separated into lower mediumstate 355 and upper medium state 357. Accordingly, hysteretic behaviorin the selection of current limit levels is provided using lower mediumstate 355 and upper medium state 357 in accordance with the teachings ofthe present invention.

[0045] In one embodiment, the current limit levels are chosen such thatthe power level delivered in different states are overlapping. Forexample, maximum power level delivered in upper medium state 357 when apattern of 5 consecutive high Enable signals 235 are followed by one lowEnable signal 235 is higher than the minimum power delivered in the highstate 359 when a pattern of 5 consecutive low Enable signals 235 arefollowed by one high Enable signal 235. Therefore, the maximum powerdelivered to the output of the power supply for the upper medium state357 current limit settings when the power supply 100 operates at amaximum on/off cycle ratio is greater than a minimum power delivered tothe output of the power supply the high state 359 current limit settingswhen the power supply operates at a minimum on/off cycle ratio.

[0046]FIG. 4 is a schematic illustrating one embodiment of state machinecircuitry 301 of switching regulator circuit 139 in accordance with theteachings of the present invention. As illustrated, in one embodiment,the inputs to state machine circuitry 301 are the Enable signal 235, theundervoltage (UV) signal 319 and the maximum duty cycle (Dmax) signal237. The outputs of state machine circuitry 301 are a one bit super highsignal 309 and a three bit signal 303 a/b/c including the high-πimsignal 303 a, the upper medium signal 303 b, and the medium signal 303c.

[0047] In operation, during power up, all the latches 457, 459, 473, and463 are reset to 0 through the UV signal 319. This places the statemachine at low state 353.

[0048] During power-up, a counter 402 is also reset to the count 0 (000in binary) because UV signal 319 is high, causing or-gate 433 to keepsignal 424 high. In one embodiment, counter 402 is a 3 bit counter. Inone embodiment, on each falling edge of Dmax 237 signal, the counter 402counts to the next number. In one embodiment, count 6 signal 479 is thedecoded output signal for this counter 402. The count 6 signal 479becomes logic high when the counter 402 counts to 6 (110 in binary). Oneway the counter can be reset to the count 0 (000 in binary) is by anychange in the Enable signal 235. If the Enable signal 235 changes fromlow to high, signal 411 from transition detector 498 will becomemomentarily high. If the Enable signal 235 changes from high to low,signal 411 will also become momentarily high. If signal 411 becomeshigh, signal 424 from OR gate 433 becomes high and resets the counterback to the count 0 (000 in binary). Thus, the counter 402 will onlykeep counting if there is a pattern of consecutive high or low Enablesignals 235.

[0049] After start-up, when the counter 402 counts to N equals 6 (110 inbinary), signal 479 becomes high, and if Enable signal 235 has been highduring all this time, AND gate 469 will change the move-up signal 408 tologic 1. When the move-up signal 408 becomes logic 1, latch 457 will setmedium signal 303 c to logic 1. At this point the state machine is inthe lower medium state 355. As soon as the transition of medium signal303 c from logic 0 to 1 is detected, signal 423 will become momentarilylogic 1, causing signal 431 and consequently signal 424 to become logic1 and resetting the counter to the count 0 (000 in binary).

[0050] When the counter counts to 6 again (110 in binary), signal 479becomes high again, and if Enable signal 235 has been high during allthis time, gate 469 will change the move-up signal 408 to logic 1. Whenthe move-up signal 408 becomes logic 1 and since medium signal 303 c isalready logic 1, latch 459 will set the high signal 418 to logic 1. Atthis point, the state machine 351 is in the high state 359. The highcurrent limit signal 303 a is only logic 1 when both the high signal 418and the Enable signal 235 are high. As soon as the transition of highstate signal 418 from logic 0 to 1 is detected, signal 420 will becomemomentarily logic 1, causing signal 431 and consequently signal 424 tobecome logic 1 and resetting the counter to the count 0 (000 in binary).

[0051] When the counter counts to 6 again (110 in binary), signal 479becomes high again, and if Enable signal 235 has been high during allthis time, gate 469 will change the move-up signal 408 to logic 1. Whenthe move-up signal 408 becomes logic 1, and since high signal 418 isalready logic 1, latch 473 will set super high signal 309 to logic 1. Atthis point the state machine circuitry 301 is in super-high state 361.

[0052] In one embodiment, to go down from super-high state 361, theEnable signal 235 has to stay low. When the counter counts to 6 (110 inbinary), signal 479 becomes high, and if Enable signal 235 has been lowduring all this time, gate 471 will change the move-down signal 407 tologic 1. When the move-down signal 407 becomes logic 1, latch 473 willreset super high signal 309 to logic 0. At this point the state machinecircuitry 301 is back in high state 359. As soon as the transition ofsuper high signal 309 from logic 1 to 0 is detected, signal 415 willbecome momentarily logic 1, causing signal 431 and consequently signal424 to become logic 1 and resetting the counter to the count 0 (000 inbinary).

[0053] When the counter counts to 6 again (110 in binary), signal 479becomes high again, and if Enable signal 235 has been low during allthis time, gate 471 will change the move-down signal 407 to logic 1.When the move-down signal 407 becomes logic 1, and if nsuper-high signal416 is logic 1, latch 459 will reset high signal 418 to logic 0. At thispoint the state machine circuitry 301 is back in upper medium state 357.As soon as the transition of high state signal 418 from logic 1 to 0 isdetected, signal 421 will become momentarily logic 1, causing signal 431and consequently signal 424 to become logic 1 and resetting the counterto the count 0 (000 in binary).

[0054] When the counter counts to 6 again (110 in binary), signal 479becomes high again, and if Enable signal 235 has been low during allthis time, gate 471 will change the move-down signal 407 to logic 1.When the move-down signal 407 becomes logic 1, and if nhigh signal 425is logic 1, latch 457 will reset medium signal 303 c to logic 0. At thispoint the state machine circuitry 301 is back to low state 353.

[0055] The medium state is additionally controlled by latch 463. Theoutput of latch 463 decides whether or not the state machine circuitry301 is in upper-medium 357 or lower-medium state 355. During power-up,latch 463 is reset. Transition from high state 359 to upper medium state357 sets the output of latch 463 to logic 1, and transition from uppermedium state 357 to low state 353 resets the output of latch 463 tologic 0. Latch 463 operation is as follows. Signal 421 will become logic1 on the high signal 418 transition from 1 to 0. This will set latch463, making the upper-medium signal 303 b logic 1. On the other hand,signal 428 will become logic 1 on the medium signal 303 c transitionfrom 1 to 0. This will reset latch 463, making the upper-medium signal303 b logic 0.

[0056]FIG. 5 is a schematic illustrating one embodiment of current limitadjust circuitry 305 of switching regulator circuit 139 in accordancewith the teachings of the present invention. As shown, a voltage dividercircuit is formed with resistor 480 transistor 485 and resistors 481,482, 483 and 484 coupled in series between drain terminal 231 andground. The inputs to current limit adjust circuit 305 are the drainsignal 231, gate signal 249, and signal 303 a/b/c. The output of currentlimit adjust circuitry 305 is signal 317. As shown in FIG. 5, whenmedium signal 303 c is logic 1, resistor 484 is shorted in the currentlimit adjust circuitry 305. When upper-medium signal 303 b is logic 1,resistors 484 and 483 are shorted in the current limit adjust circuitry305. When high-πim signal 303 a is logic 1, resistors 484, 483 and 482are shorted in the current limit adjust circuitry 305. The moreresistors are shorted, the lower the voltage at signal 317 becomesrelative to the drain voltage at drain terminal 231, thus adjusting orselecting the current limit setting.

[0057] FIGS. 6-9 are timing diagrams illustrating waveforms of variousembodiment of switching regulator circuit 139 operating in variousstates of a state machine circuitry 301 with varying current limitlevels in accordance with the teachings of the present invention. Asshown in FIG. 6, at time T0, the state machine circuitry 301 is in thelow state 353. Accordingly the current limit for the drain currentIDRAIN 255 is 0.4 πim-max with Enable signal 235 equal to 1. After apattern of N equals 6 Enable signals 235 equal to 1 for the preceding Nequals 6 drive signal cycles, state machine circuitry 301 transitions tolower medium state 355 at time T1. Accordingly the current limit for thedrain current IDRAIN 255 is 0.5 πim-max with Enable signal 235 equalto 1. After a pattern of another N equals 6 Enable signals 235 equal to1 for the preceding N equals 6 drive signal cycles, state machinecircuitry 301 transitions to high state 359 at time T2. Accordingly thecurrent limit for the drain current IDRAIN 255 is πim-max with Enablesignal 235 equal to 1. After a pattern of another N equals 6 Enablesignals 235 equal to 1 for the preceding N equals 6 drive signal cycles,state machine circuitry 301 transitions to super high state 361 at timeT3. Accordingly the current limit for the drain current IDRAIN 255 isπim-max with Enable signal 235 equal to 1 and 0.5 πim-max with Enablesignal 235 equal to 0.

[0058] As shown in FIG. 7, at time T4, the state machine circuitry 301is in the super high state 361. Accordingly, the current limit for thedrain current IDRAIN 255 is 0.5 πim-max with Enable signal 235 equal to0. After a pattern of N equals 6 Enable signals 235 equal to 0 for thepreceding N equals 6 drive signal cycles, state machine circuitry 301transitions to high state 359 at time T5. Accordingly, the cycles indrive signal 249 are skipped with Enable signal 235 equal to 0. After apattern of another N equals 6 Enable signals 235 equal to 0 for thepreceding N equals 6 drive signal cycles, state machine circuitry 301transitions to upper medium state 357 at time T6. Accordingly, thecycles in drive signal 249 are skipped with Enable signal 235 equal to0. After a pattern of another N equals 6 Enable signals 235 equal to 0for the preceding N equals 6 drive signal cycles, state machinecircuitry 301 transitions to low state 353 at time T7. Accordingly, thecycles in drive signal 249 are skipped with Enable signal 235 equal to0.

[0059] As shown in FIG. 8, at time T8, the state machine circuitry 301is in the low state 353. Accordingly, the current limit for the draincurrent IDRAIN 255 is 0.4 πim-max with Enable signal 235 equal to 1.After a pattern of N equals 6 Enable signals 235 equal to 1 for thepreceding N equals 6 drive signal cycles, state machine circuitry 301transitions to lower medium state 355 at time T9. Accordingly, thecurrent limit for the drain current IDRAIN 255 is 0.5 πim-max withEnable signal 235 equal to 1. After a pattern of another N equals 6Enable signals 235 equal to 1 for the preceding N equals 6 drive signalcycles, state machine circuitry 301 transitions to high state 359 attime T10. Accordingly, the current limit for the drain current IDRAIN255 is πim-max with Enable signal 235 equal to 1 and cycles in drivesignal 249 are skipped with Enable signal 235 equal to 0. After apattern of N equals 6 Enable signals 235 equal to 0 for the preceding Nequals 6 drive signal cycles, state machine circuitry 301 transitions toupper medium state 357 at time T11. Accordingly, the current limit forthe drain current IDRAIN 255 is 0.7 πim-max with Enable signal 235 equalto 1.

[0060] It is appreciated that because of the hysteretic nature of theupper and lower medium states 359 and 357, state machine 351 moves upfirst to high state 359 before moving back down to upper medium state357. Stated differently, state machine 351 transitions from lower mediumstate 355 directly to high state 359 without transitioning through uppermedium state 357. Therefore, the current limit of upper medium state 357is not selected when transitioning from lower medium state 355 to highstate 359.

[0061] As shown in FIG. 9, at time T12, the state machine circuitry 301is in the high state 359. Accordingly, the cycles in drive signal 249are skipped with Enable signal 235 equal to 0. After a pattern of Nequals 6 Enable signals 235 equal to 0 for the preceding N equals 6drive signal cycles, state machine circuitry 301 transitions to uppermedium state 357 at time T13. Accordingly, the current limit for thedrain current IDRAIN 255 is 0.7 πim-max with Enable signal 235 equal to1 and the cycles in drive signal 249 are skipped with Enable signal 235equal to 0. After a pattern of another N equals 6 Enable signals 235equal to 0 for the preceding N equals 6 drive signal cycles, statemachine circuitry 301 transitions to low state 353 at time T14.Accordingly, the current limit for the drain current IDRAIN 255 is 0.4πim-max with Enable signal 235 equal to 1. After a pattern of N equals 6Enable signals 235 equal to 1 for the preceding N equals 6 drive signalcycles, state machine circuitry 301 transitions to lower medium state355 at time T15. Accordingly, the current limit for the drain currentIDRAIN 255 is 0.5 πim-max with Enable signal 235 equal to 1.

[0062] It is appreciated that because of the hysteretic nature of theupper and lower medium states 357 and 355, state machine circuitry 301moves down first to low state 353 before moving back up to lower mediumstate 355. Stated differently, state machine circuitry 301 transitionsfrom upper medium state 357 directly to low state 353 withouttransitioning through lower medium state 355. Therefore, the currentlimit of lower medium state 355 is not selected when transitioning fromupper medium state 357 to low state 353.

[0063] In the foregoing detailed description, the method and apparatusof the present invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A switching regulator, comprising: a power switchcoupled between first and second terminals, the first terminal to becoupled to an energy transfer element of a power supply and the secondterminal to be coupled to a supply rail of the power supply; a drivesignal generator circuit coupled to a third terminal to receive afeedback signal representative of an output of the power supply, thedrive signal generator to generate a drive signal coupled to controlswitching of the power switch responsive to the feedback signal, thedrive signal generator circuit to selectively disable each on period ofthe drive signal in response to the feedback signal to regulate theoutput of the power supply; and a current limit circuit coupled to thepower switch and the drive signal generator circuit to control the drivesignal to limit a current flow through the power switch, the currentlimit circuit having a plurality of current limit settings for the powerswitch selected responsive to the feedback signal.
 2. The switchingregulator of claim 1 wherein the plurality of current limit settings forthe power switch includes a finite number of current limit settings forthe power switch.
 3. The switching regulator of claim 1 wherein thedrive signal generator circuit is coupled to selectively maintain an ontime of a current cycle of the drive signal and not allow an on time ofa next cycle of the drive signal in response to the feedback signal. 4.The switching regulator of claim 1 wherein the drive signal generatorcircuit is coupled to maintain an on time of a current cycle of thedrive signal until a current flowing through the power switch reaches aselected one of the plurality of current limit settings for the powerswitch.
 5. The switching regulator of claim 1 wherein the drive signalgenerator circuit comprises a state machine coupled to select among theplurality of current limit settings of the current limit circuitresponsive to a pattern of feedback signal values for a plurality ofpreceding drive signal cycles.
 6. The switching regulator of claim 5wherein the pattern of feedback signal values for a plurality ofpreceding drive signal cycles includes consecutive feedback signalvalues within one of two ranges for a plurality of N drive signalcycles.
 7. The switching regulator of claim 1 wherein a maximum powerdelivered to the output of the power supply for one of the plurality ofcurrent limit settings of the current limit circuit when the powersupply operates at a maximum on/off cycle ratio is greater than aminimum power delivered to the output of the power supply for a nexthigher one of the plurality of current limit settings of the currentlimit circuit when the power supply operates at a minimum on/off cycleratio.
 8. The switching regulator of claim 1 wherein the energy transferelement comprises a transformer.
 9. The switching regulator of claim 8wherein a flux density of the transformer is such that audio noiseproduced by the transformer is not audible when operating in an audiofrequency range at a lowest one of the plurality of current limitsettings of the current limit circuit.
 10. The switching regulator ofclaim 9 wherein an upper end of the audio frequency range isapproximately 20 kHz.
 11. The switching regulator of claim 5 wherein thestate machine is coupled to operate in one of a plurality of states toselect the plurality of current limit settings for the power switch. 12.The switching regulator of claim 11 wherein the plurality of statesincludes a first state, a second state and a third state and wherein theplurality of current limit settings for the power switch includes afirst current limit, a second current limit and a third current limit,wherein the first current limit is selected when the state machineoperates in the first state, wherein the second current limit isselected when the state machine operates in the second state and whereinthe third current limit is selected when the state machine operates inthe third state, wherein the second current limit is greater than thefirst current limit and the third current limit is greater than thesecond current limit.
 13. The switching regulator of claim 12 whereinthe state machine is coupled to transition directly from the first stateto the third state in response to the pattern of feedback signal valuesfor the plurality of preceding drive signal cycles.
 14. The switchingregulator of claim 12 wherein the state machine is coupled to transitiondirectly from the third state to the first state in response to thepattern of feedback signal values for the plurality of preceding drivesignal cycles.
 15. The switching regulator of claim 12 wherein theplurality of states includes a fourth state in which the drive signalgenerator circuit is disabled from selectively disabling each on periodof the drive signal in response to the feedback signal to regulate theoutput of the power supply.
 16. The switching regulator of claim 15wherein the plurality of current limit settings includes a first currentlimit setting and a second current limit setting, the current limitcircuit to limit the current flow through the power switch to the firstcurrent limit setting when the feedback signal is within a first rangeof two ranges, the current limit circuit to limit the current flowthrough the power switch to the second current limit setting when thefeedback signal is within a second range of two ranges.
 17. A powersupply, comprising: an energy transfer element having an energy transferelement input and an energy transfer element output coupled to an outputof the power supply; a power switch coupled between the energy transferelement input and a supply rail of the power supply; a drive signalgenerator circuit coupled to receive a feedback signal representative ofthe output of the power supply, the drive signal generator to generate adrive signal coupled to control switching of the power switch responsiveto the feedback signal, the drive signal generator circuit toselectively disable each on period of the drive signal in response tothe feedback signal to regulate the output of the power supply; and acurrent limit circuit coupled to the power switch and the drive signalgenerator circuit to control the drive signal to limit a current flowthrough the power switch, the current limit circuit having a pluralityof current limit settings for the power switch selected responsive tothe feedback signal.
 18. The power supply of claim 17 wherein the energytransfer element comprises a primary winding coupled to the energytransfer element input and a secondary winding coupled to the energytransfer element output, wherein the current limit circuit is coupled tolimit the current flow through the primary winding.
 19. The power supplyof claim 17 wherein the energy transfer element comprises a transformer.20. The power supply of claim 18 wherein the drive signal oscillates ata frequency greater than an audio frequency range if a flux density ofthe energy transfer element is at a value that causes the transformer togenerate audible noise when driven by the drive signal.
 21. The powersupply of claim 20 wherein an upper limit end of the audio frequencyrange is approximately 20 kHz.
 22. The power supply of claim 17 whereinthe plurality of current limit settings for the power switch includes afinite number of current limit settings for the power switch.
 23. Thepower supply of claim 17 wherein the drive signal generator circuitcomprises on/off control circuitry coupled to generate the drive signal,the on/off control circuitry coupled to selectively disable the onperiod of each cycle of the drive signal responsive to the feedbacksignal.
 24. The power supply of claim 23 wherein the drive signalgenerator circuit disables a cycle of the drive signal from beinggenerated responsive to the feedback signal.
 25. The power supply ofclaim 17 wherein the drive signal generator circuit comprises a statemachine coupled to select among the plurality of current limit settingsof the current limit circuit responsive to a pattern of feedback signalvalues for a plurality of preceding drive signal cycles.
 26. Theswitching regulator of claim 25 wherein the pattern of feedback signalvalues for a plurality of preceding drive signal cycles includesconsecutive feedback signal values within one of two ranges for aplurality of N drive signal cycles.
 27. A method for regulating a powersupply, comprising: switching a power switch coupled in series with anenergy transfer element of the power supply with a drive signal tocontrol an amount of power delivered to an output of the power supply;selectively disabling a cycle of the drive signal from being generatedresponsive to the output of the power supply; selecting one of aplurality of current limit settings responsive to the output of thepower supply; and limiting a current flow through the power switch inresponse to the selected one of the plurality of current limit settings.28. The method for regulating the power supply of claim 27 whereinselecting one of the plurality of current limit settings compriseschanging states in a state machine responsive to output values of theoutput of the power supply for a plurality of preceding drive signalcycles.
 29. The method for regulating the power supply of claim 28wherein changing states in a state machine responsive to output valuesof the output of the power supply comprises counting a number ofconsecutive output values of the output of the power supply within afirst range of output values of the output of the power supply for aplurality of N drive signal cycles
 30. The method for regulating thepower supply of claim 27 wherein selectively disabling a cycle of thedrive signal from being generated comprises: maintaining an on time of acurrent cycle of the drive signal; and disabling an on time of a nextcycle of the drive signal in response to the output values of the outputof the power supply.
 31. The method for regulating the power supply ofclaim 27 further comprising maintaining a frequency of the drive signalabove an audio frequency range for flux density values of the energytransfer element that are greater than a flux density threshold value.32. The method for regulating the power supply of claim 30 wherein theflux density threshold value is a lower end of a range of flux densitythreshold values of the energy transfer element that result in ageneration of audible noise from the energy transfer element if operatedwithin the audio frequency range.